The present invention relates to a flight control system, and more particularly to a flight control system for a rotary wing aircraft.
Flight control systems for rotary wing aircraft typically employ: a cyclic stick for commanding the aircraft's pitch and roll, pedals for commanding directional yaw, and a collective stick to control the vertical rate. Fly by wire aircraft typically utilize active controllers which provide force cueing capability along with providing the pilot commands to the control system. The active controller denotes the ability to directly provide cueing forces to a control input inceptor grip from an electronically computed source related to the effect the controller is having on the aircraft. The force cueing aspects of such active controllers are typically complex, require fault criticality considerations and management, and are therefore more expensive than a simple conventional displacement stick type controller.
The collective control stick typically is connected to an actuation device that provides minimal tactile feedback. The manual displacement of the control stick may not provide sufficient or accurate perception of the extent of collective pitch input, the effect on the aircraft vehicle loads and systems or of the amount of collective pitch reserve available. The reduced physical cues provided by a displacement type collective control stick may not be particularly well suited to aircraft which must perform in a flight profile where tactile feedback is of significant importance, such as nap-of-the-earth flight. The pilot may thereby rely in part upon secondary cues within the cockpit, such as torque or collective position indicators.
Conventional displacement type collective sticks typically utilize trim motors connected in series to a spring to control the stick position. When the conventional displacement type collective stick is moved away from a reference set by the trim motor, a return force is provided by the spring. The return force may be accompanied by a damping force device.
The conventional displacement type collective stick is pivotally connected to a fixed point within the aircraft cockpit which facilitates movement about a large radius such that tactile feedback is essentially linear. This essentially linear motion reduces pilot fatigue and facilitates operation of the various push-button controls positioned on the face of the collective grip. The spring gradient may be accompanied by an initial trim detent or breakout force and damping force. The spring and detent forces enable return to trim if the pilot relaxes his force on the stick and also enables the automatic flight control system to control the position of the stick when the pilot does not provide the positioning function.
While conventional displacement type collective sticks offer the pilot the desired physical displacement and rate damping cues, modern aircraft tend to desire additional control stick cues that are difficult if not impossible to implement with the trim spring plus damping mechanical approach. To improve the force cueing capability, force sensors have been added to the conventional displacement type collective stick actuation. Although relatively effective, such conventional displacement type collective sticks may have disadvantageous weight characteristics. Furthermore, it may be difficult to command an artificial cueing force directly that may not be related to the position of the control stick. This is why the trend is toward the active controller methodology for Fly-By-Wire systems where cueing force can be directly commanded by the electronic control system.
Another significant method of cueing force is the use of a force-generating hydraulic cylinder which is commanded by a digital computer. While the cylinder does provide generalized force cueing capability, it too may have disadvantageous weight characteristics and may introduce a high force and high rate failure characteristics which must be accommodated through redundant hardware design and software criticality considerations.
In one conventional trim motor and spring trim system, a motor positions one end of a trim gradient spring while the other end connects to the stick through an electrically controlled engage clutch. The nature of the engage clutch is that no slippage is permitted such that the pilot moves the stick by compressing/stretching the spring. Since the spring stores energy when compressed and since there is typically a high gear ratio to the motor and clutch to reduce torque design parameters, clutch release dissipates stored energy thru inertia acceleration of the gear train such that the result causes some stick jump as perceived at the pilot's hand. Furthermore, the compression of the relatively soft spring may complicate or prevent the command of significant cueing forces to the pilot because a large deflection of the spring or large detent spring preload would be required. Both of these may lead to undesirable operational characteristics and severely limit the cueing capability of the conventional trim motor-spring type actuation systems.
Modern control systems also apply a technology of “active controller” systems as part of the pilot control input inceptor system. Such active controllers have the capability to control position and force applied to the control stick and meet the requirements for generalized force cueing. However, such systems may have large force and motion failure modes which necessitates that hardware and software design criticality be addressed. Therefore, while the stick-actuator-inceptor hardware is effective, a higher level of criticality and therefore redundancy in the control hardware and software system design may be necessitated.